Microencapsulation has been studied in a wide variety of fields including recording materials such as pressure-sensitive recording papers, drugs, perfumes, agricultural chemicals, adhesives, foods, dyes, solvents, rust inhibitors, liquid crystals and health-promoting foods. Various microcapsules have already been put to practical use or are now under tests for their commercial applications.
A number of techniques has already been proposed especially on the microencapsulation of hydrophobic materials (both oily and solid materials). Among such techniques, the coacervation technique (phase separation technique) making use of gelatin is now applied on a commercial scale, principally, for pressure-sensitive copying paper.
However, microcapsules formed in accordance with the complex coacervation technique which uses gelatin and an anionic polyelectrolyte are accompanied inter alia by the following problems:
(1) Since the complex coacervation technique is difficult to provide on principle a microcapsule slurry having such a high solid concentration as 20% or more, resulting microcapsules require a high shipping cost and evaporation of a great deal of water is indispensable for their application in pressure-sensitive copying paper. There is still great room for improvement from the standpoint of working speed and energy cost.
(2) Microcapsules undergo large changes in both quality and price because their walls are made of natural materials.
(3) Microcapsules cannot be stored over prolonged periods since they are susceptible of putrefaction and coagulation.
Therefore, there has been a strong demand for the development of improved microcapsules in view of the quality and manufacturing cost of pressure-sensitive copying paper.
As improved techniques capable purportedly of satisfying such demands, there have been proposed in Japanese Patent Laid-open Nos. 9079/1976, 84881/1978, etc. production processes of high-concentration microcapsule slurries, each of which processes makes use of a urea-formaldehyde resin or melamine-formaldehyde resin obtained by the in-situ polymerization process as a film-forming material. Various improvements have also been proposed in addition to the above-mentioned processes.
In a process making use of a melamine-formaldehyde resin as a film-forming material, it is proposed for example in Japanese Patent Laid-open No. 84881/1978 to use, as an anionic polyelectrolyte, ethylene-maleic anhydride copolymer, methyl vinyl ether-maleic anhydride copolymer, propylene-maleic anhydride copolymer, butadiene-maleic anhydride copolymer, vinyl acetate-maleic anhydride copolymer and polyacrylic acid. These copolymers are however accompanied by such problems that they require high temperature and long time for their dissolution and due to high viscosity levels of resulting microcapsule slurries, they can hardly provide microcapsule slurries having high solid contents.
On the other hand, Japanese Patent Laid-open Nos. 49984/1979 and 47139/1980 disclose some examples of microencapsulation in a system of a styrene-maleic acid copolymer or in a system of a combination of a styrene-maleic acid copolymer and another maleic acid copolymer. However, styrene-maleic acid copolymer has poor solubility in water. Thus, its dissolution has to be effected at a high temperature for long time in the presence of an alkaline material. At a pH level of 4 or lower, the viscosity of the system is increased and its state of dispersion is destroyed due to deposition of the polymer. Therefore, it is difficult to remove formaldehyde at the acidic side. In addition, the above microencapsulation processes provide microcapsule slurries having relatively high viscosity levels.
Japanese Patent Laid-open No. 58536/1981 proposes to emulsify and disperse a core material, which is to be enclosed in microcapsules, in an aqueous solution of the homopolymer of a compound containing neither phenyl group nor sulfophenyl group but containing a sulfo group, specifically, sulfoethyl acrylate or methacrylate, sulfopropyl acrylate or methacrylate, maleinimido-N-ethanesulfonic acid or 2-acrylamido-2-methylpropanesulfonic acid, or a copolymer of such a compound and an alkyl acrylate, hydroxyalkyl acrylate or N-vinylpyrolidone; and then to add a melamine-formaldehyde pre-condensate in an amount conforming with the degree of condensation either continously or little by little. The above-proposed microencapsulation process is however not preferred at all from the viewpoint of workability for the following reasons. Namely, the dispersion system remains unstable after the addition of the melamine-formaldehyde pre-condensate but before the formation of microcapsule walls where the core material is in an oily form. Thus, resulting oil droplets tend strongly to agglomerate into larger droplets unless the dispersion system is kept under very strong stirring conditions. Accordingly, it is difficult to control the diameters of emulsified droplets to desired sizes. Furthermore, the system may be gelated in its entirety or may develop coagulated particles of the melamine-formaldehyde pre-condensate unless the pre-condensate is charged little by little with special care over a long period of time. Moreover, this this microencapsulation process is incapable of providing any microcapsule slurry having a high solid content in excess of 50 wt. %.
It has also been proposed in Japanese Patent Laid-open No. 155636/1981 to form films as microcapsule walls by preparing an aqueous medium containing a polymer, which does not by itself have dispersion stability for a liquid material as a core material at acidic pH levels but owing to an interaction with a melamine-formaldehyde initial condensation product, does form a material capable of imparting dispersion stability of the liquid material, and the melamine-formaldehyde intial condensation product, adding the core material to form a stable dispersion after formation of the dispersion-stabilizing material, and then condensing the melamine-formaldehyde initial condensation product with an acid catalyst.
However, it is indispensable for the above microencapsulation process to cause the melamine-formaldehyde intial condensation product and the polymer to undergo partial condensation at a low temperature and for a long period of time prior to the incorporation of the core material in order to form the dispersion-stabilizing material for the core material. The core material is then emulsified and dispersed, and the system is thereafter heated to effect the condensation. Unless conditions for the partial condensation of the melamine-formaldehyde initial condensation product in the presence of the polymer are controlled precisely, the resulting emulsion will have such poor stability that the resulting microcapsules will have irregular sizes of the viscosity of the microcapsule slurry will tend to increase to a considerable extent. The above microencapsulation process involves another problem from the viewpoint of controlling the process steps. Furthermore, it cannot provide any microcapsule slurry having a solid content of 55 wt. % or higher.
On the other hand, use of urea-formaldehyde resin as a wall-forming material for microcapsules is disclosed as microencapsulation processes making use of hydrophobic materials obtained by the in-situ polymerization process in Japanese Patent Laid-open Nos. 9079/1976, 84882/1978, 84883/1978, 53679/1979, 85185/1979, 47139/1980, etc.
The production processes disclosed in the above publications are also accompanied with various problems. In Japanese Patent Laid-open No. 9079/1976, is disclosed a microencapsulation process for a hydrophobic material formed by condensation of urea and formaldehyde while using an ethylene-maleic anhydride copolymer, methyl vinyl ether-maleic anhydride copolymer or polyacrylic acid as an anionic water-soluble polymer. This process is however accompanied by such problems that it takes long time at a high temperature to dissolve such a polymer in water and the thus-prepared microcapsule slurry has an extremely high viscosity. On the other hand, Japanese Patent Laid-open No. 84883/1978 discloses to produce microcapsules by subjecting a urea-formaldehyde initial condensation product such as dimethylol urea, methylated dimethylol urea or the like to condensation in the presence of the above-mentioned water-soluble polymer. This microencapsulation process is however accompanied by the same drawbacks as that proposed in Japanese Patent Laid-open No. 9079/1976.
Reference may also be made to Japanese Patent Laid-open No. 53679/1979, in which there is proposed a process for producing microcapsules equipped with urea-formaldehyde resin walls in the presence of a styrene-maleic anhydride copolymer. Microcapsules are however caused to deposit at a low pH level of 4 or less when such a styrene-maleic anhydride copolymer is use. Therefore, this microencapsulation process is accompanied by such imminent drawbacks that it requires severe conditions for the production of microcapsules and the dissolution of such a polymer in water requires not only high temperature and long time but also an addition of an alkaline material.
In addition, Japanese Patent Laid-open No. 51238/1981 discloses use of a styrenesulfonic acid type polymer upon preparation of melamine-formaldehyde resin walls. When this process was applied to microcapsules having walls made of a urea-formaldehyde resin, it was difficult to obtain dense capsule walls and the whole system tended to coagulate or gelate by slight changes in conditions. Therefore, this process could not become a stable process for preparing microcapsules with good workability. On the other hand, Japanese Patent Laid-open No. 14942/1983 discloses to use a copolymerized anionic polymer consisting of three or more monomers including at least (A) acrylic acid, (B) a hydroxyalkyl acrylate or hydroxyalkyl methacrylate and (C) styrenesulfonic acid upon producing microcapsules having walls made of a melamine-aldehyde or urea-aldehyde polycondensation product in the presence of an anionic water-soluble polymer. However, it does not contain any disclosure about the preparation process of the copolymerized water-soluble polymer, and according to the reexamination of the present inventors, such copolymer caused gelation when polymerizing or did not undergo copolymerization to any sufficient extent and hence still contained unreacted monomers. Therefore, many of such copolymers were unsuitable for actual applications and were never satisfactory.